Ancestral women moved among stable male bands?

Recent research regarding two long-extinct hominin species has received a lot of press lately. Much of it has ridiculously over-interpreted or misinterpreted the published findings. What Sandi Copeland and her collaborators argue officially (article here) is that these species preferred for females to move into new groups after childhood, much like “chimpanzees, bonobos and many human groups”; males, in contrast, mostly remained part of the band into which they were born.

Maybe so – but this impressive research is preliminary. It represents only an initial step in demonstrating that this pattern occurred. And, despite some fun headlines, the species involved are not necessarily our ancestors. So here’s a list of concerns to temper the fanfare:

The researchers assumed that small-toothed individuals were female and that large-toothed ones were male. Given the probable sexual dimorphism (size differences) in these species, this seems reasonable. But many things that seem reasonable work out differently in practice. At least one other expert has highlighted this as iffy: “their assignment of sex is methodologically risky.”

Even if the methodology were unimpeachable, the research depends on a statistical analysis of only 19 fossils. This is too few to generalize about two species who existed for hundreds of thousands of years. The possibility that the pattern they see is the result of chance is too high. As researchers find more fossils to test, this objection will fade away.

The researchers do not make universal claims about the movement of females and males, but news articles, and especially headlines, suggest that all women moved away from their birthplace and that no men did so. Depending on how they assigned individuals to groups, only 50-75 percent of females had died away from their birthplace, and 11-17 percent of males. So females might have been just as likely to stay put as to move.

The research depends on ratios of strontium isotopes in teeth. These chemical signatures differ based on geological conditions in a given area, and they develop in the first eight years of life. If there were a clear pattern of some individuals staying in place and others moving after childhood, we would expect to see a clear division between the local ratio and non-local ones. Instead, the researchers found a smooth progression of ratios (see their Figure 2); I’m curious about their explanation. Also, some non-local areas have strontium signatures that overlap with the signature of the area where the fossils were found. Presumably this could hide movement between these areas by males or females.

This raises a further issue: could members of these species really have lived in such a small area? Areas with different strontium ratios are less than 2 miles away. Are there large, foraging primates who would not travel that far in childhood?

Of the two species studied, Australopithecus africanusmight be our ancestor, or this name might refer to two species, one of which might be our ancestor. The other species in the study, Paranthropus robustus, almost certainly isn’t our ancestor. So headlines regarding this research that refer to “human ancestors” are misleading.

The researchers don’t help matters when their article refers to “many human groups” who display a “similar” pattern of female mobility, along with chimps and bonobos. They don’t acknowledge that, as other researchers recently noted, “the human pattern of residency is so variable that it counts as a pattern in itself.” That might help explain their evidence of mobile males and sedentary females.

Despite the above, this is truly a creative and promising approach to investigating long-past movements, and the addition of more cases might well establish the authors’ explanation beyond a reasonable doubt – and possibly for species that are more clearly in our ancestral line.